1. Field of the Invention
The present invention relates to lead frame material used for semiconductors, integrated circuits (IC) or the like, and a method for producing the same.
2. Discussion of Background
Heretofore, various copper alloys have been used as lead frame material for semiconductors, integrated circuits or the like in view of their high electroconductivity and heat-dissipativity.
FIG. 1 is a perspective view illustrating a lead frame. In FIG. 1, the lead frame 1 comprises a die-pad part 2 on which a semiconductor chip is placed; an inner lead part 3 connected with the semiconductor chip by a wire (Au wire or Cu wire); and an outer lead part 4 bonded with a printed-circuit board.
In this structure, an Ag-plating 5 having a thickness of at least 4 .mu.m is generally applied on the surface of the inner lead part 3 and the die-pad part 2 on which a semiconductor chip is placed. Without the Ag-plating 5 on the inner lead part 3, the bonding strength of a wire bonding which connects the semiconductor chip and the inner lead part 3 by an Au or Cu wire is not stable and poor in reliability. The die-pad part 2 does not require the Ag-plating 5, but the Ag-plating 5 is applied also on the die-pad part 2 because it is difficult to apply the Ag-plating 5 on the inner lead part 3 only.
However, the above mentioned Ag-plating process has the following disadvantages.
1 A manufacturing cost becomes high because expensive Ag is plated in a large thickness of at least 4 .mu.m.
2 When the Ag-plating is carried out, a Cu- or Ni-underplating is applied, but the underplating is applied on the total surface of the lead frame 1 after being formed, and it is therefore necessary to remove the underplating of the outer lead part 4 after molding. Sn-or solder-plating is applied on the outer lead part 4, but when the Sn- or solder-plating is applied on the underplating, i.e. without removing the underplating, it is liable to be peeled off at the interface with the underplating and the reliability of plating is lowered.
Thus, there is a demand for a material suitable for direct wire bonding which connects an Au- or Cu-wire directly with the inner lead part of the surface of the matrix material without conducting thick Ag-plating.
Presently, it is partly practiced to connect a Cu-wire directly with a lead frame. For example, Japanese Unexamined Patent Publication No. 17841/1989 discloses a lead frame material having a Cu-Ag alloy layer of a thickness of at least 0.5 .mu.m on its surface, which can be used for direct wire bonding.
However, its use is presently limited to semiconductors or IC which do not require high reliability, and more highly reliable lead frame material is demanded for most of semiconductors or IC which require high reliability.